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Unit-4: History of the Atomic models Name:_____________________ Period__ 9-30-2019

TABLE-1 Atomic models:

Atomic model Model diagram Model Description

1. Democritus model:

~ 500 BC

• Atom is the smallest unit of matter • Atoms are ________, all atoms are homogeneous (identical),

cannot be destroyed (indestructible) and cannot be ___________ down into smaller units (indivisible)

2. Dalton’s model

1897

• Atoms are (extremely small) the smallest units of matter • Atoms are _____________ and indivisible • All atoms are NOT homogeneous – atoms of different elements are

different from one another. But, atoms of one element are identical in size, mass and chemical properties to each other.

• ___________ ___________ reactions, atoms of different substances combine in simple whole number ratios to form new compounds. Atoms are not created or destroyed but only rearranged during chemical reactions - LAW OF MULTIPLE PROPORTIONS. Also proves LAW OF MASS CONSERVATION.

3. J. J. Thomsen model (Plum pudding model)

1904

• All atoms contain _______________ charged particles called electrons (CATHODE RAY TUBE experiments)

• Since atoms are _____________, there are positively charged particles in the atoms as well

• Positive and negative particles are spread throughout the atom like a plum pudding

4. Rutherford model (NUCLEAR model)

1911

• There is a tiny, dense, positively charged nucleus at the center of

the atoms (GOLD-FOIL experiment) • _____________ are located around the nucleus • The atom contains mostly empty space •

5. Bohr model

1913

• Electrons are located around the nucleus in specific, ______________ energy levels (or shells /orbits= like planets around the sun)

• The ____________ between energy-levels is called a quantum • Electrons can _________ from one quantum to another when they

gain or lose energy (Colors of Fireworks!)

6. Quantum mechanical model (the current model)

1926 - present

• Proposed by Erwin Schrödinger as a modification of the Bohr model • Electrons are not located around the nucleus in specific, spherical

energy levels but are located as a cloud around the ___________ (ELECTRON CLOUD)

• Electrons are found in ORBITALs = 3-D areas around the nucleus with a high ______________ of finding an electron

• There are s, p, d and f orbitals with different 3-D shapes • Electrons are both particles and waves

Fe H

H Fe

Orbits (energy level)

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Practice problems: For each statement below, write true or false.

1. Ancient philosophers regularly performed controlled experiments.

2. Philosophers formulated explanations about the nature of matter based on their own experiences.

3. Both Democritus and Dalton suggested that matter is made up of atoms.

4. Dalton’s atomic theory stated that atoms separate, combine, or rearrange in chemical reactions.

5. Dalton’s atomic theory stated that matter is mostly empty space.

6. Dalton was correct in thinking that atoms could not be divided into smaller particles.

7. Dalton’s atomic theory stated that atoms of different elements combine in simple whole-number ratios to form compounds.

8. Dalton thought that all atoms of a specific element have the same mass.

9. Democritus proposed that atoms are held together by chemical bonds, but no one believed him.

10. Dalton’s atomic theory was based on careful measurements and extensive research.

Answer the following questions explaining your answers:

1. What are similarities between the Democritus and Dalton models of the Atom?____________________ ___________________________________________________________________________________________

2. What are some differences between the Democritus and Dalton models of the Atom?_______________ ____________________________________________________________________________________________

3. According to Dalton’s research on the Law of Mass Conservation, which new product (C) is formed from reactants in Box A and B combine?

4. Cathode rays were discovered by:____________________________________________

*In a neutral atom: number (MASS) of protons = number of electrons = ATOMIC NUMBER In any atom: ATOMIC MASS = total number (MASS) of Protons + total number (MASS) of Neutrons Thus, number (Mass) of Neutrons = ATOMIC MASS - ATOMIC NUMBER • Electrons are the only sub atomic particles that participate in CHEMICAL REACTIONS • Neutrons and Protons participate in NUCLEAR REACTIONS

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5. Charged particles: Negative charges are repelled by each other. Positive charges are repelled by each other, as well. But opposite charges are attracted to each other. Opposite charges

6. Cathode is the _______________electrode and anode is the ______________ electrode in an electric circuit. Electric currents are made up of moving ______________. Since electrons are ____________ charged, they emerge from the ______________ of an electrical circuit and travel towards the ________ which has a positive charge.

*Cation = atom that lost 1 or more electrons (Cat-ions are Paw-sitively charged) *Anion = atoms that have gained 1 or more electrons (Negatively charged)

7. Draw the direction of movement of the cathode ray in the experiment below:

8. What evidence from the Cathode Ray Tube (CRT) experiments did J.J. Thompson use to determine that electrons were negatively charged? ___________________________________________________ _________________________________________________________________ _________________________________________________________________

9. How did Thomson determine that all types of matter contained electrons?_______________________ ________________________________________________________________________________________

10. Model of Rutherford’s gold foil experiment: If Alpha particles that hit the nucleus are deflected, what is the net charge of an alpha particle?________

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11. Why did Ernest Rutherford use a thin gold foil to test if J.J. Thompson’s atomic model was accurate?

12. What evidence did Rutherford have to conclude that the atom was mostly empty space?_____________

___________________________________________________________________________________ 13. What evidence from the gold-foil experiment did Rutherford use to conclude that the nucleus was tiny

and was located at the center of the atom?________________________________________________ ___________________________________________________________________________________

14. Protons was discovered by:_____________________________________________________________ 15. Neutrons were discovered by:___________________________________________________________ 16. What was discovered by Millikan’s Oil Drop experiment?_____________________________________ 17. According to Millikan’s calculations, the electron is almost ______ - times __________ in mass than a

proton or a neutron. The mass of an atom is calculated by adding the total masses of protons and neutrons in the nucleus. Electrons are left out of this calculation because their mass is considered to be ___________________________.

18. What was a key flaw of Rutherford’s model of the atom?

19. How did Neils Bohr save the Rutherford model?

20. Neil’s Bohr wanted to explain why metallic elements like iron (Fe) ________ in bright colors when they were heated. To explain this, modified Rutherford’s Atomic model to include electron ___________. According to his theory, electrons stay on specific ___________ paths (ORBITs) around the nucleus. This is much like the way planets orbit the sun!!! The distance between orbitals is called a _____________. When electrons gain or lose a __________ quantum of ____________ they can “jump” from one orbital to another, while __________ that energy as heat or light. Bohr was on the right track towards explaining the atomic model, but his model did not explain the structure of atoms that had more than ___ electrons.

21. Erwin Schrödinger challenged the Bohr model of the atom by stating that ____________ act like waves and not particles as Bohr and others suggested. Heisenberg suggested both Bohr and Schrödinger were right as _____________ act as both particles and waves. This gave birth to the quantum mechanical model of the atom. In this model, electrons are located in a cloud-like area around the nucleus called an “electron-_________.” Schrödinger used _____________ calculations to describe the PROBABILITY (likelihood) of finding an electron in a certain position inside the atom using 3-dimensional (3-D) ORBITALS. These orbitals (s, p, d and f orbitals that have different shapes).

22. What is meant by wave and particle duality of the electrons?

23. What is an Atom according to the current model of the atom?__________________________________ ___________________________________________________________________________________

24. What is an element?__________________________________________________________________

25. Label the mass number and the atomic number on the following atom: a)___________________

b) ___________________ 1224 Mg

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26. Determine the number of protons, electrons, and neutrons for each isotope described below. a. An isotope has 21 neutrons and mass number 40. b. An isotope has atomic number 19 and mass number 39. c. An isotope has 14 electrons and a mass number of 28. d. An isotope has 21 neutrons and a mass number of 41. e. An isotope has 22 neutrons and a mass number of 41.

27. Which of the isotopes in problem 21 are isotopes of the same element? Identify the element (s). ______________________________________________________________________________________________

28. Based on the quantum mechanical model of the atom shown, fill in the blanks.

29. Write each isotope below in symbolic notation. Use the periodic table to determine the atomic number of each isotope. 1) neon-22 2) Carbon-14 3) Ca- 41

4) cesium-133 5) cesium-132 6) uranium-234

7) Thorium -232 8) Radium 226 9) Radon -222

30. All atoms have ____ subatomic particles: Electrons, protons and the neutral, ____________. All of these subatomic particles are identical between elements. But different elements have different numbers of protons, neutrons and electrons. It is the number of __________ present in an atom that gives its chemical and physical traits unique to each element. All chemical reactions occur due to the _____________ or ____________ of electrons between atoms. When atoms gain or lose electrons, it does not become a different element. But, if you add or remove protons from an atom, this atom will become a different type of atom (element).

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Unit-4 Homework-6 Due_________ Name__________________________ Period_____ Defining the Quantum mechanical model of the Atom (Modern accepted model)

A. TABLE-2 Properties of sub-atomic particles:

Particle Symbol Location Relative Charge

Relative Mass Actual mass in grams (g)

1. Proton

2. Neutron

3. Electron 1/1840

B. Periodic Table: How to read and interpret sections of the Periodic table:

(a)________________________

(b)________________________

(c)________________________

(d)________________________

C. Atomic Number is equal to: _______________________________________________________________

D. Mass Number (ATOMIC MASS) is equal to: ____________________________________________________

E. What is an isotope?_______________________________________________________________________

_______________________________________________________________________________________

F. Define the term, atomic mass unit (a.m.u):____________________________________________________

_______________________________________________________________________________________

G. Use the Periodic Table of Elements to complete the table below: TABLE-2

Element (symbol)

Atomic Number

Number of protons

Number of electrons

Number of neutrons

Name of element

a) Pb 82 ______ ______ ______ ______

b) ______ ______ 8 ______ ______ ______

c) ______ ______ ______ 30 ________ ________

Hydrogen 1 H

1.008